1. Field of the Invention
The present invention relates to a process for producing an ink composition which is preferably used in ink jet recording, and an ink composition prepared thereby.
2. Background Art
An ink prepared by adding a dye as a water-soluble colorant component to an aqueous medium is generally known as an ink for ink jet recording. On the other hand, however, there are not a few dyes having poor lightfastness, waterfastness and other various properties. Utilization of an ink composition comprising a pigment as a colorant component has recently been studied from the viewpoint of improving the above properties.
Since the pigment is generally insoluble in water, utilization thereof in water-based ink compositions requires that the pigment is mixed with a polymer called a "dispersant" and the like to prepare a mixture which is then stably dispersed in water followed by preparation of an ink composition using the dispersion.
Stable dispersion of the pigment in an aqueous system requires studies on type and particle diameter of pigments, type of dispersants, dispersion methods and the like. Up to now, various dispersion methods and ink compositions for ink jet recording have been proposed in the art.
Japanese Patent Laid-Open No. 157954/1994 discloses that a pigment and a polymeric dispersant are charged into a two-roll milling (two-roll mill) device to prepare a pigment dispersion that is then dispersed in an aqueous carrier medium to prepare an ink composition for ink jet recording.
Japanese Patent Publication No. 55548/1986 discloses that a polymer (a dispersant) previously prepared by polymerizing a monomer is dispersed together with a pigment and a water-soluble polyhydric alcohol (ethylene glycol) in a ball mill, a sand mill or the like to prepare a pigment dispersion for the preparation of an ink composition for ink jet recording.
Japanese Patent Laid-Open No. 227668/1992 discloses a process for producing an ink composition, comprising the steps of: providing a previously prepared block polymer as a dispersant, using this dispersant in combination with a pigment, a water-soluble organic solvent, such as diethylene glycol, and water in a horizontal Mini mill or the like to prepare a dispersion; and diluting the dispersion to prepare an ink composition.
In ink jet recording, an improvement in response frequency of a recording head is necessary from the viewpoint of improving recording speed. To this end, the amount of the ink ejected is preferably small. For this, preferably, a larger dot diameter is provided using a smaller amount of the ink ejected. On the other hand, however, particularly in a blotted recorded image (solid image), a larger amount of ink should be ejected in order to eliminate the creation of white streaks caused by lack of dot diameter.
A commonly used method for solving this problem is to add a surfactant or a glycol ether to an ink to impart a penetrating property. In the case of pigment-based inks, for example, Japanese Patent Laid-Open No. 147861/1981 proposes utilization of triethylene glycol monomethyl ether as a pigment.
Further, the ink for ink jet recording has generally been prepared by dispersing a pigment together with a dispersant to prepare a pigment dispersion and adding various components to the pigment dispersion.